Which of the following is correct about the molecularity of a reaction?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → It may be the same as or different from the order of the reaction
Molecularity is the number of reacting species that must collide at the same time in an elementary reaction. Order is the sum of the powers of the concentration terms in the experimentally found rate law.
Same: for an elementary step, and for the slowest step of a complex reaction, the molecularity equals the order.
Different: order is experimental and can be zero or a fraction, which a molecularity never is. For example, a zero order reaction still involves at least one reacting molecule.
Why the other options are incorrect
Option (1) It can be zero or a fraction — Incorrect. Molecularity counts colliding particles, so it is a whole number (1, 2 or 3); it is the order that can be zero or a fraction.
Option (2) It is equal to the order of the reaction — Incorrect. This holds only for elementary reactions, not always.
Option (3) It applies to complex reactions as well as to elementary reactions — Incorrect. Molecularity applies only to elementary reactions; for a complex reaction it has no meaning. It is order that applies to both.
NCERT (2026-27 edition):
3.2.4 Molecularity of a Reaction – “(i) Order of a reaction is an experimental quantity. It can be zero and even a fraction but molecularity cannot be zero or a non integer.”
“(iii) For complex reaction, order is given by the slowest step and molecularity of the slowest step is same as the order of the overall reaction.”